The present invention relates generally to the fabrication of ceramic sheets, and more particularly to an apparatus for applying a force to these ceramic sheets while they are being fired to prevent shrinkage, distortion, and camber.
Ceramic sheets are of particular importance in the electronics industry for the packaging/mounting of semiconductor integrated devices and other elements. The fabrication of such ceramic substrates generally is well known and entails mixing of the ceramic with a binder and various solvents, casting the "green" sheets of this ceramic mixture, drying the green sheets, blanking and punching via holes in the sheets, screening metallurgy into the via holes, stacking and laminating these sheets, firing these laminated sheets, and finally sintering the resulting laminated structure. The details for these various processing steps are set forth in U.S. Pat. Nos. 3,423,517; 3,723,276; 4,340,436; 4,234,367; and 4,301,324.
Of particular concern in processing these ceramic structures is the shrinkage and distortion which the ceramic structure undergoes during sintering. The cohesive forces which operate during sintering cause reproducible shrinkage in the x-y plane, non-linear x-y plane shrinkage, as well as via bulge and curvature in the z direction, referred to as camber. It is particularly important to control the x-y plane shrinkage because the dimensions for the x-y plane determine the location for the chips to be disposed on the ceramic substrate. Additionally, precise x-y plane dimensional control is essential to permit the use of automated wire bonders and automated testers.
Various methods for reducing the effects of x-y plane shrinkage and z-direction distortion have been proposed in the art. A typical distortion reduction method is disclosed in U.S. Pat. No. 4,009,238 which teaches the use of applied pressure by means of compression rams to the substrate surface during the sintering step, which is the processing step when most of the x-y plane shrinkage and distortion occurs. The application of this weight or force on the substrate during sintering operates to balance or counteract the cohesive forces of sintering and forces any shrinkage to occur only in the z direction (the thickness direction for the substrate). Accordingly, ceramic substrates sintered with the above-described loading thereon are flat with minimal x-y plane shrinkage. However, the standard method for applying force or loading to the substrate during sintering is to use a large weight with an attendant large thermal mass on the substrate. Such weights are cumbersome to move on a standard ceramic substrate conveyor belt. Additionally, because of their large thermal mass, these weights require a significant amount of time and energy to heat to the required sintering temperature. Typically, the required time for sintering is doubled with the use of such weights disposed on the substrate. A further problem with such weights is that their use requires the reservation of a certain volume above the substrate, thereby limiting the thickness of the ceramic product which can be sintered in a given furnace. Yet a further problem with the use of such weights is that they have a high center of gravity, thereby causing stability problems during conveyance to and from the sintering furnace.
The invention as claimed is intended to remedy the above-described problems with current techniques for ceramic substrate loading during sintering.
The advantages offered by the present invention are that ceramic substrate loading can be obtained with a low thermal mass device which can be heated quickly in a sintering oven, thereby significantly reducing the sintering time and energy required. Likewise, the device of the present invention has a very small volume and a low center of gravity, so that it does not limit the thickness of the product being sintered, and does not cause stability problems during the conveyance of the product to and from the sintering oven.